Bacterial infections remain a leading cause of worldwide morbidity and mortality. While antibiotics are often safe and efficacious, there is widespread concern over bacterial strains that have become resistant to classic antibiotic treatment regimens. In individuals infected with resistant strains, antibiotic administration results in incomplete and ineffective treatment necessitating additional treatment along with propagation of the resistant strains. Preventative measures and alternative treatments that do not rely on antibiotics are therefore desirable.
The respiratory and gastrointestinal tracts are frequent sites of bacterial infections in normal individuals. The normal respiratory tract has natural clearance mechanisms that help to prevent bacterial colonization. These mechanisms include the presence of a mucus gel that acts as a barrier to bacterial invasion, the propulsive forces of the cilia on the epithelial lining of the airways, and the secretion of antibacterial humoral factors such as the secretory immunoglobulins IgA and IgM, the proteins lactoferrin, betalysin and fibronectin, complement components and the enzyme lysozyme. Of these antimicrobial factors, lysozyme is the best established antimicrobial substance in airway secretions.
In certain individuals such as those who are immunocompromised, who are in less than optimal health, who lack fully functional immune systems such as neonates or geriatric patients, or who suffer from a disease affecting the respiratory tract such as cystic fibrosis or the gastrointestinal tract such as ulcerative colitis or sprue, bacterial infections may have severe consequences leading to serious illness or even death. For example, production of altered mucus in cystic fibrosis patients leads to dilation of the exocrine ducts, destruction of acinar tissue, and replacement of the destroyed tissue by fibrous connective tissue. Involvement of the lungs leads to pneumonia and bronchiectasis. The paucity of systemic dissemination of infection in these patients, even in the presence of substantial bacterial colonization of the lungs, indicates that their systemic immunity is essentially intact, yet they are susceptible to pulmonary infections. These patients often die in their teens or early twenties from terminal lung infections in spite of aggressive antibiotic therapy.
Human lysozyme is an naturally occurring enzyme that is known to exhibit bactericidal activity in vitro and thus would appear to be a promising way to prevent and/or treat bacterial infections. Lysozyme is a small (15 kilodaltons), basic protein that is produced in most tissues. It is secreted and is present in most body secretions such as mucus. In the lungs, immunohistochemical methods have localized lysozyme to the bronchial serous submucosal glands, alveolar macrophages and lamellar bodies in Type II alveolar epithelial cells. Approximately 80% of lysozyme secreted into the airway comes from the mucosal layer of the upper airways.
In vitro, lysozyme has been demonstrated to act independently to cause bacterial death. It is known that one way lysozyme kills bacteria is by hydrolyzing the glycosidic bond between C-1 of N-acetylmuramic acid and C-4 of N-acetylglucosamine in the bacterial polysaccharide cell wall. Lysozyme can also kill bacteria by acting synergistically with other proteins such as complement or antibody to lyse bacterial cells. Lysozyme, produced by polymorphonuclear leukocytes such as neutrophils, inhibits chemotaxis of polymorphonuclear leukocytes and limits the production of oxygen free radicals following an infection. This limits the degree of inflammation, while at the same time enhances phagocytosis by these cells. Lysozyme is also probably involved in the response of airway tissue to injury.
While the antibacterial effects of lysozyme in vitro have been well documented, there has heretofore been no way to exploit these effects of lysozyme for in vivo use. Previous reports furthermore implied that sustained lysozyme administration would be harmful.
Surfactant protein-B (SP-B) is one of the protein components of pulmonary surfactant. Pulmonary surfactant, a complex mixture of phospholipids and proteins, is synthesized and secreted by alveolar type II epithelial cells, a specialized exocrine cell. Normal respiratory function requires pulmonary surfactant for maintenance of alveolar patency.
A method and composition for the prophylaxis and/or treatment of bacterial infections, particularly respiratory bacterial infections as frequently occurs in patients with cystic fibrosis, is thus desirable.